1. Field of the Invention
This invention pertains to apparatus and processes used in treating flexible sheet material, and more particularly to vacuum guide/fluid applicator combinations used to apply fluid treating compositions to moving flexible sheet material, such as paper web, during papermaking and finishing operations.
2. Description of Background Information
The application of fluid treating compositions, including coatings, to flexible sheet materials has generally been accomplished using application devices, such as gapped rolls, squeeze rolls, curtain bars, doctor knives or spray nozzles, which generally involve the application of significant quantities of liquid to the flexible sheet material. Drying operations subsequent to the treating step are generally required to evaporate the applied liquid leaving an applied amount of solid treating agent. For example, such procedures have been used to apply paper treating agents during papermaking or finishing operations, but with recognized limitations.
Squeeze rolls involve contacting the flexible sheet material with a composition containing treating agent in the form of either a liquid or foam, immediately prior to passing the flexible sheet material through the rolls. Limitations in the use of treating agent which can be applied by squeeze rolls arise due to various limitations in their properties. The treating agent compositions must generally be low-viscosity, low-solids compositions since strong hydraulic forces generated between squeeze rolls operating at high speeds separate the rolls leading to loss of control in the amount of composition applied and other problems. Studies show that fluid penetration mechanisms govern pick-up such that a strength gradient from the sheet surface to the sheet center can develop, unless saturation occurs. The significant increase in moisture content of the sheet from squeeze roll applications requires added drying steps and apparatus, significantly increasing costs and limiting processing speed.
Spraying treating agent compositions onto flexible sheet material also has limited utility. Generally, only low-viscosity liquids can be utilized. Limitations in the uniformity of application of treating agent compositions also exist. Sensitivity to spray nozzle design and performance, as well as inherent limitations in spray configurations and overlap, leads to inefficient distribution. Spraying often provides uneven deposition, particularly for wide sheet material, such as used in papermaking operations. Spraying is also sensitive to air currents which may affect uniformity of deposition. Certain treating agents may not be sprayed for environmental reasons due to risks of being spread through the air.
Various high-solids compositions have been developed attempting to alleviate drying restrictions enabling increased application rates. Foamed compositions have been used since volumetric expansion of the composition for obtaining uniform coverage is provided by using gas in place of liquid reducing the need for drying operations.
These foam systems, however, are not fast breaking but are designed to provide foamed treating compositions used as a pond or reservoir using traditional fluid application means, such as air-knives, rolls, brushes or the like, followed by subsequent disintegration of the foam using squeeze rolls, doctoring blades or the like. The effective use of such foam application procedures requires foamed treating compositions of sufficient stability such that when exposed to air the foam does not randomly or unevenly break down to liquid causing uneven wetting of the substrate and/or uneven doctoring by blade or squeeze rolls used in the traditional application procedures. Furthermore, recycling of such compositions to maintain composition consistency is usually difficult and impractical.
Developments in treating operations have led to high efficiency, short-dwell treating devices which permit the use of high solids treating compositions. Such systems reduce the amount of liquid applied to the flexible sheet material, thereby reducing the energy required to evaporate the liquid during subsequent drying operations, often enabling higher application rates providing increased productivity and lower costs. Such short-dwell treating operations enable the application of reduced amount of treating compositions, thereby further reducing costs. Additionally, the short-dwell applications permit higher application rates by reducing or eliminating the need for liquid reservoirs or "ponds" having the previously described disadvantages.
Short-dwell fluid treating applications are described in, for example, "Surface Treatment Under Vacuum", by R. Akesson, Paper Technology and Industry May/June, 1977; "14 Years of Progress in Coatings--from Billblade to Twinblade" by W. Williams, Paper, May 5, 1980; "Twostream Coater-Versatile New Tools for Coating in Spotlight", S. Westergard, Paper Age, May 1984. Additional short-dwell fluid treatment applications are described in U.S. Pat. No. 3,941,902 (Wennerblom et al.) and U.S. Pat. No. 4,023,526 (Ashmus et al). When using the procedure described in the Ashmus et al. patent, the impingement of the fluid treating composition upon the substrate can lift the substrate away from the foam applicator nozzle lips resulting in leakage and spillage of foam.
The critical feature of short-dwell treating applications involves the requirement for intimate contact between the flexible sheet material and the orifice of the fluid applicator to enable satisfactory application of the fluid treating composition to the flexible sheet material without spillage while providing uniform deposition of treating composition.
Typically, short-dwell fluid treating applications have involved placing the flexible sheet material between rolls or similar guiding devices which are disadvantageous in being bulky, requiring precision speed control and being of varying diameter causing an uneveness of the flexible sheet material. Alternatively, an expensive "bow" roll can be used to provide uniform flatness of wide flexible sheet material contacting the fluid applicator.
There is therefore a need to provide a fluid applicator having the advantages of short-dwell fluid application operations, but which avoids the bulky, expensive or inefficient means for providing the requisite intimate contact with the orifice of short-dwell fluid applicators. Such fluid application operations should operate at relatively high substrate speeds and be useful in a continuous application operation, such as papermaking or finishing operations. It would be desirable if the treating application would introduce a minimum of induced tensile drag on the flexible sheet material by providing an adjustable application of force to generate sheet smoothness. Ideally, treatment operations should maintain intimate contact between the flexible sheet material and the fluid applicator despite small but significant height variations across the width of the fluid applicator.